The response of human T lymphocytes to porcine vascular endothelium will be a major factor determining the outcome of clinical tissue or organ xenotransplantation. This response will likely be determined by a combination of direct and indirect recognition of donor peptides expressed on donor or recipient MHC molecules. Studies of in vitro responses indicate the xenogenic human anti-porcine response is surprisingly stronger than the human alloresponse. By contrast, in vivo experimental models show that the xenoresponse is significantly weaker than the alloresponse. We hypothesize that some protein on human endothelial cells is required for in vivo but not in vitro responses and that this protein(s) expressed by porcine cells to a lesser degree or not recognized by human T cells in vivo. The investigators have developed the capacity to genetically transduce endothelial cells with viral vectors with high efficiency. This together with the development of a synthetic collagen/fibronectin gel microvessel system that can be introduced into huPBL/SCID beige mouse models offers a means to define properties of xenorecognition in vivo. The specific aims of this proposal are: (1) to firmly establish the synthetic microvessel using normal porcine aortic endothelial cells (PAEC) and Bcl-2 transduced Paec, (2) to characterize the interaction of human PBMC with porcine PAEC-derived microvessels in vivo, (3) to genetically modify PAEC to overexpress factors that affect T cell endothelial cell interactions and characterize the consequences in vivo and in the synthetic microvessel system in vivo, and (4) to utilize the information to evaluate the responses in two additional models in the huPBL-SCID being mouse: the porcine arterial graft and the porcine skin graft. These experiments will provide critical information regarding the recognition properties of T cells in a xenograft model that is essential for xenotransplantation.